In recent years, due to the development of power electronics, apparatuses controlled by a power device, such as IGBT and MOS-FET are rapidly increasing. In particular, power-device controlling is rapidly proceeding in transportation apparatuses such as electric railway transportation apparatus and vehicles. Furthermore, in accordance with increased concern about environmental problems, for example, electric cars, and hybrid cars using both a gasoline engine and an electric motor, have begun to be placed on the market, so that the demand for power modules to be mounted thereon is expected to increase. For such use, particularly high reliability is required in view of the object of its use.
In a conventional power module, in order to prevent the temperature of a semiconductor device from raising above a predetermined temperature by causing the heat generated in the semiconductor or the like to dissipate outside, the structure thereof is generally such that a semiconductor device is mounted on a ceramic circuit board of aluminum oxide (Al2O3), silicon nitride (Si3N4), aluminum nitride (AlN), or the like by soldering, and is soldered to a heat sink which is composed of a metal such as copper (Cu) or aluminum (Al).
In the case of such a structure, however, it may occur that a solder layer between the ceramic circuit board and the heat sink cracks when subjected to a heat cycle during the operation of the semiconductor device, or to changes in the temperature of the operating environment, or the like. The solder layer cracks due to a thermal stress generated by a difference between the thermal expansion of a ceramic substrate and that of the heat sink. The presence of cracks in the solder layer (hereinafter simply referred to as the “solder cracks”) lowers the dissipation of the heat generated in the semiconductor device and elevates the temperature of the semiconductor device. As a result, the semiconductor device becomes deteriorated, and the reliability of the power module is lowered as a whole.
As semiconductor apparatus becomes highly integrated and requires higher power, higher heat dissipation is demanded, and it is desired to make the solder free of lead in view of the environmental pollution. However, a so-called lead-free solder has a problem that the reliability thereof is lower than that of Pb—Sn solders which are currently used widely, although the heat conductivity of the lead-free solider is high.
In order to avoid these problems, it is being studied to use in the heat sink an Al—SiC composite material or a Cu—Mo composite material, which has such a thermal expansion coefficient that is closer to that of the ceramic substrate. However, in comparison with a conventional metal heat sink, such a heat sink has the problems that the heat sink has to be fabricated by a special process and the processing step and the surface treatment step therefor are more costly and such a heat sink is by far more expensive.
On the other hand, there has been conducted a trial of directly bonding a heat sink and a ceramic circuit board by using a brazing filler metal instead of a solder in order to avoid the occurrence of solder cracking and to improve on heat dissipation (refer to JP-A-9-97865 and JP-A-10-270596).
In this case, however, such problems are caused that debonding of a junction interface and cracking of the ceramic circuit board become apt to occur due to the thermal stress generated by a difference in thermal expansion between the ceramic circuit board and the metal heat sink, and that the stress applied to the solder that bonds a semiconductor device and the ceramic circuit board is increased, so that cracking of the solder under the semiconductor device becomes apt to occur more easily. Furthermore, the shape and warp of the heat sink may greatly change under a heat history during the assembly process of the power module or under the actual use thereof, so that there may be a case where a problem is caused during the assembling of the power module, and the lowering of heat dissipation performance takes place due to a decrease in the close contact between the heat sink and a heat dissipation block.
The present invention was made in view of the above-mentioned circumstances, and an object of the present invention is to provide a module which has a module structure comprising a ceramic circuit board and a metal heat sink, the module structure being such that the changes in the shape thereof are small even under a heat history during the assembly thereof and under the actual use thereof, and the assembling thereof is easy, abnormalities such as the debonding at the junction interface, the cracking of a ceramic substrate, the formation of cracks in a solder layer, and the like, are difficult to occur, with excellent heat dissipation performance, and with high reliability that can be maintained over a long period of time.